In a testing process of a TFT substrate, a testing apparatus including a testing modulator and a charge-coupled device (CCD) is used to perform the test at a position of 15-20 microns above the substrate. It is necessary to align the center positions of test modulator and the CCD of the testing apparatus in order to ensure a correct testing result of the TFT substrate. Otherwise, if there is an offset between the center positions of the CCD and the modulator, testing result of the TFT substrate will have error. Therefore, it is necessary to calibrate the testing apparatus of the TFT substrate before testing the TFT substrate.